When a person is confined to bed, soft tissue is compressed between the skeleton and the supporting surface. Care is usually taken to provide a deformable mattress but, nevertheless, high local pressures occurring in the deformed tissue will compress blood vessels and tissue damage may result. A patient resting on a normal hospital bed will experience local pressures of the order of 150 mm Hg (2.9 psi). The blood pressure through the capillary vessels of the skin and underlying tissue is generally accepted as being 26 mm Hg (0.503 psi), but this figure may be considerably reduced for an ill patient. When contact pressures exceed this value, blood flow is stopped, resulting in transient damage and, finally, deep penetrating necrosis of tissue, muscle and bone. Skin may also be damaged by shear stresses resulting from friction between the skin and the supporting structure. Such stresses are a function of the local pressure and the area of contact.
Some improvements may be gained by the combination of the airfilled mattress with an air permeable top surface, as illustrated and described by Howarth in U.S. Pat. No. 3,778,851.
Subsequent to the inception of work upon air cushion vehicles, it became apparent that the air cushion principle could be used to provide the required evenly distributed, low pressure support desired for bedridden patients, especially those suffering from burns. In pursuit of this objective, a number of patents have been filed. For example, L. A. Hopkins and A. R. Tripp in U.S. Pat. No. 3,340,550 and L. A. Hopkins in U.S. Pat. No. 3,340,551 illustrate and describe apparatus for supporting a patient's body on a gaseous cushion. Examples of such apparatus have been built and are in operation for the treatment of burns in the United Kingdom. While they are effective, they do leave substantial areas for improvement, including (1) they are very expensive; (2) they have high power consumption; (3) the method of providing the seal for the air cushion supporting the patient requires individual adjustment of a large number of elements to suit the particular shape of the patient; and (4) their size, shape, weight and required ancillary equipment does not permit their being placed into conventional hospital rooms.
Subsequent to the development of the above apparatus, effort was directed toward the reduction of the power requirements and a so-called "Low Air Loss Bed" was developed. It is described in a United Kingdom Patent No. 1,273,342, and a similar apparatus is described by Dr. Scales in U.S. Pat. No. 3,822,425. This low air loss system does not support the whole area of the patient's body without contact, as does the higher air loss system, but it does reduce the higher body contact pressure by interposing an air film between the mattress and the patient's body at those points of higher body contact pressures. These earlier inventions are representative of the prior ways to support the human body by a gaseous cushion.
The embodiments hereinafter illustrated and described are distinguishable in the many ways they support the human body, in whole or in part. These systems are designed to be used with current hospital bed frames and/or other bed frames to provide the advantages of essentially contact-free, uniformly low pressure patient support at lower costs, greater ease of maintenance and sterilization and wider versatility in use. In systems for whole body support, they have the feature of automatically conforming to the variable contours, shapes and cross-sections of patients of differing sizes and weights and providing contact-free fluid support. In other smaller embodiments, these features are also realized when smaller selected portions of a body are supported.